Components of various structures are manufactured using metal injection molding (MIM) techniques. MIM processes employ granular feedstock that includes powdered metal. Such feedstock is injected into a mold to form a green part, which is substantially geometrically similar to the final component, although the green part may be oversized relative to the final component to account for shrinkage during the subsequent sintering step. Next, the green part is subjected to de-binding, for example in a thermal or solvent-based process, to remove the binder and form a brown part. The brown part is then sintered at high temperatures to form the final, substantially metallic component.
It is often necessary and/or desirable to quantify certain material properties of MIM-manufactured components by testing coupons, formed in the same way and from the same materials as the components of interest. Such test coupons typically have a reduced central cross-sectional area and are sometimes referred to as “dog bone” coupons. When injecting feedstock through the mold during the MIM process to produce a test coupon, the reduced central cross-sectional area acts as a restriction that inhibits proper flow of the feedstock through the reduced-area section of the coupon. If feedstock injection pressure is increased to overcome the restriction, in some cases feedstock material is pushed through the restriction too quickly, shearing the binder away from the feedstock. If the binder is not uniformly distributed throughout the green test coupon after the injection-molding process is completed, the coupon may become warped during sintering. On the other hand, if the feedstock is pushed through the restriction too slowly, the binder cross-links before injection is completed, and the feedstock material tends to solidify, preventing further injection.
Due to the fact that green parts are relatively brittle, a long, narrow green part having a reduced-area central section may not survive typical mold-ejection techniques without damage. Mold-release agents are typically not used in MIM processes because of potential feedstock contamination problems. Accordingly, an increased number of ejector pins may be incorporated into the mold to provide better distribution of the ejection force, experienced by the green test coupon during the mold-release step, in an attempt to reduce the possibility of damaging the coupon as it is released from the mold. However, since the long, narrow shape and the reduced central cross-sectional area of the coupon require elevated injection pressures during the MIM processes, binder may flow into spaces between the ejector pins and the mold, causing the ejector pins to stick to the mold and become incapable of ejecting the coupon from the mold. Moreover, a long, narrow green part having a reduced-area central section, such as a green test coupon formed by an MIM process, as described above, may warp at the elevated temperatures, associated with sintering.
A green test coupon formed in an MIM process may also have undesirable “flash,” i.e., ridges of excess material, formed by feedstock, seeping into the mold parting lines during the MIM process. Removal of flash from a long, narrow green part using conventional manual techniques introduces the risk of damaging the coupon.